Two types of highly repeated DNA segments in the genome of the African green monkey (Cercopithecus aethiops) are being studied: (1) Certain satellite DNAs are characterized by long tandem repetitions of species specific sequences and centromeric location. Earlier work indicated that the organization of one such monkey satellite, deca-satellite, is highly polymorphic in individual members of the species and that the amounts of deca-satellite and Alpha-satellite, the major monkey satellite, vary (independently) in individual genomes. In an effort to understand the maintenance of such extensive but variable DNA sequences, analysis of junctions between satellite and unique genomic sequence has been initiated; several such DNA segments have been cloned and characterized. In the junctions, satellite is jointed to low copy number DNA sequences. Homologous low copy number sequences also join a species specific satellite in the human genome. (2) The major primate family of highly repeated long interspersed DNA sequences (called LINE-1, previously KpnI family) includes several thousand 6 kbp long units that terminate in an A-rich stretch. In addition there are more than 10,000 dispersed copies of truncated, rearranged and deleted LINE-1 segments in primate genomes. A 6 kbp sequence compiled from independently cloned and squenced human and monkey LINE-1 subsegments contains open reading frames totalling about 3.5 kbp. The region containing the open reading frame is conserved in rodents. On the basis of these data we proposed that one or more LINE-1 family members may be functional genes and encode a protein. Therefore the existence of polyadenylated cytoplasmic RNA that might be messenger RNA for the putative genes was investigated. Such a transcript, which represents the sense-strand of the LINE-1 sequence, was detected in a human teratocarcinoma tissue culture cell line. The abundance of the transcript varies markedly with previously described variations in the phenotype of these cells. Antibody prepared against a synthetic peptide whose structure as predicted from the open reading frame, detects an appropriately sized polypeptide in extracts of the teratocarcinoma cells.